Subsea drilling operations may experience a blow out, which is an uncontrolled flow of formation fluids into the drilling well. Blow outs are dangerous and costly. Blow outs can cause loss of life, pollution, damage to drilling equipment, and loss of well production. To prevent blowouts, blowout prevention (BOP) equipment is required. BOP equipment typically includes a series of functions capable of safely isolating and controlling the formation pressures and fluids at the drilling site. BOP functions include opening and closing hydraulically operated pipe rams, annular seals, shear rams designed to cut the pipe, a series of remote operated valves to allow controlled flow of drilling fluids, and well re-entry equipment. In addition, process and condition monitoring devices complete the BOP system. The drilling industry refers to the BOP system in total as the BOP Stack.
The well and BOP connect to the surface drilling vessel through a marine riser pipe, which carries formation fluids (e.g., oil, etc.) to the surface and circulates drilling fluids. The marine riser pipe connects to the BOP through the Lower Marine Riser Package (“LMRP”), which contains a device to connect to the BOP, an annular seal for well control, and flow control devices to supply hydraulic fluids for the operation of the BOP. The LMRP and the BOP are commonly referred to collectively as simply the BOP. Many BOP functions are hydraulically controlled, with piping attached to the riser supplying hydraulic fluids and other well control fluids. Typically, a central control unit allows an operator to monitor and control the BOP functions from the surface. The central control unit includes hydraulic control systems for controlling the various BOP functions, each of which has various flow control components upstream of it. An operator on the surface vessel typically operates the flow control components and the BOP functions via an electronic multiplex control system.
Certain drilling or environmental situations require an operator to disconnect the LMRP from the BOP and retrieve the riser and LMRP to the surface vessel. The BOP functions must contain the well when a LMRP is disconnected so that formation fluids do not escape into the environment. To increase the likelihood that a well will be contained in an upset or disconnect condition, companies typically include redundant systems designed to prevent loss of control if one control component fails. Usually, companies provide redundancy by installing two separate independent central control units to double all critical control units. The industry refers to the two central control units as a blue pod and a yellow pod. Only one pod is used at a time, with the other providing backup.
While the industry designed early versions of the pods to be retrievable in the event of component failure, later versions have increased in size and cannot be efficiently retrieved. Further, while prior art systems have dual redundancy, this redundancy is often only safety redundancy but not operational redundancy, meaning that a single component failure will require stopping drilling operations, making the well safe, and replacing the failed component. Stopping drilling to replace components often represents a major out of service period and significant revenue loss for drilling contractors and operators.
The industry needs a simple and cost effective method to provide added redundancy and prevent unplanned stack retrievals. The industry needs an easily retrievable system that allows continued safe operation during component down time and integrates easily and quickly into existing well control systems. The industry needs a simpler, economic, and effective method of controlling subsea well control equipment.